1. Field of the Invention
This invention relates generally to novel methods and apparatus for analog to digital conversion and more specifically to methods of digitizing variables by utilizing digitized time feedback and to systems utilizing the methods.
2. Description of Prior Art
Conditions which can be measured, (e.g., temperature, voltage, pressure, weight, distance, velocity, capacitance, etc.), are often digitized so that the measure can be expressed as discrete steps or digits. The first step in digitizing often includes the use of a transducer, a device that will convert energy from one form to another. For example, a piezoelectric crystal can be used to convert pressure variations into an analog voltage and a thermistor can be used to generate an analog voltage as a function of temperature. These voltages can then be digitized by an analog-to-digital converter. Other transducers, however, can more directly generate a digital output. For example, an interferometer can convert a displacement into a changing optical interference pattern that can then be converted into a pulsing, and hence digital, voltage by a photocell.
A popular form of analog-to-digital converter utilizes a ladder of matched resistors to divide either the input voltage or a reference voltage into a series of levels and arrive at a digitized or quantized representation of that voltage. When used in these devices, feedback is in the form of a voltage.
Voltage-to-frequency converters have a frequency output which is continuously variable. To be more useful, this output can be counted by a binary counter gated by a fixed clock or the output can be used to gate a fixed clock being used to drive a counter. Voltage-to-time converters, such as integrators, can be used in a similar manner as a substitute for the voltage-to-frequency converters.
A third form of converter is the delta-modulator which generates a single weighed digital pulse train with the polarity of the pulses dependent on a difference signal. This difference signal is created by subtracting the input voltage from a feedback voltage. The feedback voltage is generated by either a charge dispensing circuit or a digital-to-analog converter.